So, I'm gonna try this mod. I have a few questions :
1. Is it reccomended for relativly new players? (E.G managed to run a goblin free fortress for 4 years. I did not, mind you, turn off invasions. I didn't realize that I couldn't acces the goblins.)
Depends what exactly you mean by new I guess but I find this mod to be great to help learn to survive a siege, you get them fast and frequent so you get to test different strategies on how to break them without risking the loss of an ancient fort, not to mention the higher difficulty..
2. Would an embark full of zombies/skeletons be a good siegebreaker? AKA will the undead attack sieges.
They'd probably attack them, I know I've had a few goblin sieges and/or ambushes show up at the same time as other enemies (as well as a few semi-megabeasts) and they all fought outside. That said, unless its been changed, the undead in general are extremely weak these days (due to a work around from them being incredibly strong) so I doubt they'd put up a meaningful fight, plus (again unless somethings changed) oftentimes you don't have a huge amount of undead on evil maps, at least not at any given moment, undead animals are like normal ones, they come and go, and other undead creatures are even less frequent (in my experience at least, used to play evil biomes all the time, not so much these days).
So I guess in short, yes they'd probably attack but no it probably won't help in a meaningful way with you surviving the siege.
-MB